(a) Field of the invention
The present invention relates to an endoscope, and more particularly to an optical system for altering the direction of the visual field in endoscopes.
(b) Description of the prior art
As one of the conventional optical systems for altering the direction of the visual field which are of such a type as mentioned above, the optical system for side viewing which is stated in, for example, Japanese Patent Preliminary Publication No. Sho 55-101239 is of the arrangement as shown in FIG. 1 that, in an optical system of the retrofocusing type endoscope, the light bundle coming from an object impinges, via a concave lens 1, onto a roof prism 2 which is constructed with a highly refractive glass block, and that the light bundle emitting from this roof prism 2 is focused on an end surface of an image guide 4 by an objective lens system 3 to be transmitted to the other end of this image guide, and it is characterized by a parallel plate 5 inserted between the concave lens 1 and the roof prism 2 based on the above-mentioned fundamental arrangement. Whereby, it is said that the adjustment of decentration or the holding of the prism can be accomplished with an enhanced precision, and also that the effective light bundle is reduced in size to achieve the realization of reduction of the outer diameter of the concave lens 1.
Also, as another example of the optical systems for altering the direction of the visual field, the optical system for oblique viewing stated in, for example, Japanese Patent Preliminary Publication No. Sho 60-140313 is constructed, as shown in FIG. 2, by arranging the light-incidence surface of the first prism 6 of the visual field direction altering means at right angles relative to the direction of the visual field, and arranging the first reflection surface 7a of the second prism 7 parallel with the axial direction of the endoscope, and also arranging the interface layer 8 of the first prism 6 and of the second prism 7 to form an acute angle with the axial direction of the endoscope, and along therewith, by using said interface layer 8 to serve as the second reflection surface 7b, and by bonding the interface layer 8 with a bonding agent. Whereby, it is said that the processing of the prisms and the fabrication of the objective lens are facilitated, and that the loss of the amount of decentration and of the amount of light due to the play among these components can be prevented.
However, in the abovesaid optical system for side viewing, a roof prism 2 is employed. Therefore, in order to secure a margin or room between the light bundle on the incidence side and the roof (Dach) surface, the distance from the end surface on the light-incidence side of the roof prism 2 up to the optical axis of the objective lens 3 and the optical axis of the image guide 4 requires to be set at a certain value or greater. For this reason, the outer diameter of the endoscope becomes as large as about 10 mm or greater, thus limiting the reduction of the size of the endoscope. Also, in the abovesaid optical system for side viewing, there is a feature that a pair of prisms 6 and 7 are used to realize an erect (correct) image without the employment of a roof prism. However, there has never been encountered in the past an example which has developed the above-mentioned feature into the side-viewing optical system for practical use.